Clinch joints for fastening sheets of deformable material such as sheet metal or the like have been used for over one hundred years. Lanced clinch sheet metal joints are used today which are very similar in design as shown in U.S. Patent No. 56,494--Gordon, issued in 1866. Over the years, clinching has experienced limited commercial utilization and currently the most popular method of fastening overlapping segments of sheet metal together is spot welding. Spot welding poses problems, particularly in certain applications such as when using dissimilar metals, metals having protective coatings, metals with different thicknesses or hard to weld materials. Clinch joints are not problem free either and whether to use a spot weld or a clinch joint must be determined on an application by application basis. Clinch joints of the lanced variety shear the metal exposing the central portion of the sheet to the atmosphere. A lanced joint is not suitable when a leak proof joint is necessary, such as where the metal sheets have corrosion resistant coatings or the joint must be pressure-tight, such as a beverage can tab top attachment.
Leak proof clinch joints are well known, but have yet to experience wide spread commercial uses. Leak proof clinch joints of a conventional design are shown in U.S. Pat. Nos. 3,359,935--Rosbottom, 3,579,809 --Wolf, 4,459,735--Sawdon, and described in detail in SAE Technical Paper, A New Mechanical Joining Technique for Steel Compared to Spot Welding, J. M. Sawhill and S. E. Sawdon, U.S. Pat. No. 830,128. Conventional leak proof clinch joints are formed by simultaneously drawing a pair of cup-shaped cavities nested one within the other between a punch and die. Once the cavities are drawn, the spot forming the central region of the cavity is squeezed axially to radially deform and interlock the nested cavities.